1. Field of the Invention
The present invention relates to reticle cassettes for carrying reticles (photomasks) in exposure apparatuses that use extreme ultraviolet (EUV) rays as a light source for exposure.
2. Description of the Related Art
Reticles are used for transferring desired patterns onto wafers in exposure apparatuses. Particles existing on the pattern surfaces of the reticles prevent high-precision exposure, and as a result, lead to defects of devices in some cases.
To solve this problem, reticles are carried while the pattern surfaces thereof are covered with transparent protective films, and the reticles are used for exposure while being covered with the protective films in known exposure apparatuses.
On the other hand, wavelengths of exposure lights have been shortened in recent years due to miniaturization of transfer patterns, and exposure lights such as F2 laser beams (157 nm) and EUV rays (5 to 20 nm) have been developed as alternatives to known exposure lights such as i lines (365 nm), KrF excimer laser beams (248 nm), and ArF excimer laser beams (193 nm). These short-wavelength exposure lights are readily absorbed into the above-described protective films, and therefore, the known method cannot be applied without modification.
Thus, methods including a step of carrying reticles while the reticles are accommodated in reticle cassettes (storage units) and a step of taking the reticles out of the reticle cassettes before exposure have been proposed. FIGS. 9A-B and 10A-B illustrate reticle cassettes disclosed in Japanese Patent Laid-Open No. 2005-123292 (corresponding to U.S. Patent Appl. No. 2005-095,829). The reticle cassette shown in FIGS. 9A-B includes a reticle-bearing portion 110 and a cover 120 such that a reticle 200 is interposed between and supported by the reticle-bearing portion 110 and the cover 120. Moreover, the reticle cassette shown in FIGS. 10A-B, which serves as a modification of the reticle cassette shown in FIGS. 9A-B, includes electrostatic adhesive portions 172 on the cover 120 such that the reticle 200 is supported by the cover 120 using the electrostatic adhesive portions. Such a reticle cassette is carried to an exposure chamber, and then the cover of the reticle cassette is removed from the reticle-bearing portion. Subsequently, the reticle 200 is taken out of the reticle cassette and then mounted on a reticle stage.
During carrying of the reticle, there is a high probability that particles are generated after the reticle is taken out of the reticle cassette.
For example, when the reticle is taken out of the reticle cassette using a robot hand, the reticle cassette and the reticle that are in contact with each other are first separated. Next, the reticle and the robot hand that are in contact with each other are separated when the reticle is transferred from the robot hand to an alignment stage for positioning the reticle. Furthermore, the reticle and the alignment stage that are in contact with each other are separated when the reticle is transferred from the alignment stage to the robot hand after positioning the reticle. Furthermore, the reticle and the robot hand that are in contact with each other are separated when the reticle is transferred from the robot hand to a reticle stage.
When the reticle is separated from substances that are in contact with the reticle, particles are generated by exfoliation, and the like. Thus, it is preferable that the number of contacts is kept to a minimum.